Oscillatory heat engines such as thermoacoustic engines or Stirling engines may be used for refrigerators or heat pumps, where the energy of gas pressure and volume oscillations is converted into the movement of heat into the engine at low temperatures and out of the engine at high temperatures. Oscillatory heat engines may also be used as prime movers, where heat flow into the engine at high temperatures and out of the engine at low temperature is converted into gas pressure and volume oscillations.
Conventional Stirling engines have long been used as prime movers that accept heat at a high temperature, reject less heat at lower temperature, and convert a large fraction of the difference into useful work in the form of pressure and volume oscillations of a working gas, such as helium. They are able to do this with high efficiency, defined as the useful work output divided by the hot heat input. Heat is exchanged between the external world and the working gas through a hot and a cold heat exchanger. A regenerator is placed between these two heat exchangers. The regenerator is a fine porous structure, such as a stack of fine metal screen, that is in intimate thermal contact with the gas. Conventional Stirling engines use two sliding pistons connected by a mechanical linkage to effect translations of the working gas through the regenerator that are properly phased with volume changes of the gas. In a common version of such engines a displacer piston translates the gas through the regenerator and a power piston takes net power from the engine by allowing expansion of the gas while the pressure is high and contraction of the gas while the pressure is low. Although Stirling engines have been under development for almost 200 years, their mechanical complexity has historically limited their commercial viability. Stirling engines with high power density typically have internal pressures in excess of 20 atmospheres, creating problems with the pressure vessel penetrations and sliding seals necessary for connecting rods and moving pistons. The mass of the moving parts also limits the operating frequency of the engine to the detriment of the engine's power density. The development of the free-piston Stirling engine eliminated the mechanical linkage between the power and displacer pistons and some of the sliding seals by resonating the pistons on gas or mechanical springs. However, the problems associated with internal sliding piston seals and the limits brought on by large piston masses remain.
The pressure and volume changes of the working gas can be considered more generally to be manifestations of sound in the gas. This acoustic point of view inspired the relatively recent invention of the thermoacoustic-Stirling engine described in U.S. Pat. Nos. 4,114,380; 4,355,517; 6,032,464 and 6,314,740 and WIPO publications WO 99/20957 and WO 02/086445 all hereby incorporated by reference for their teachings of such engines. In the acoustic point of view, the function of the flywheel in the original Stirling engine is accomplished instead by resonance—the repetitive oscillation of energy back and forth between potential and kinetic forms. Potential energy is stored in the compression or expansion of stiffness elements which may take several forms, such as metal springs or gaseous volumes that act as springs. Kinetic energy is stored in the oscillatory motion of masses, also of several forms, such as the motion of a mass of a fluid or a mass of solids. Resonance may be established between discrete lumped stiffness and mass elements, as in the case of a solid block on a coil metal spring or of the so-called Helmholtz acoustic resonator; or resonances may be established among distributed elements, each element having both stiffness and mass-like properties, as in the case of a ringing tuning fork or the acoustic resonances of a gas filled tube; or resonance may occur in a continuum between these lumped and distributed extremes.
The thermoacoustic-Stirling engine shares with the free-piston Stirling engine the use of gas springs, but its much simplified mechanical design employs acoustic networks to form acoustic masses and springs that mimic the function and motion of mechanical pistons, thereby completely eliminating all pistons, mechanical linkages and sliding seals. In some forms of the thermoacoustic-Stirling engine, a resonant tube of gas, approximately the length of half a wavelength of sound, functions as the power piston.
An earlier version of thermoacoustic engine utilizes standing waves and imperfect thermal conduction between the working gas and the porous medium, called a stack in this type of engine rather than a regenerator, to achieve the proper phasing between gas motion through the stack and expansions and contractions of the gas. Examples of patents teaching this type of technology are U.S. Pat. Nos. 4,398,398; 4,489,553; 4,722,201 hereby incorporated by reference. Thermoacoustic standing wave engines share with thermoacoustic-Stirling engines the advantages of eliminating the moving pistons of conventional or free-piston Stirling devices, and are even simpler in their construction. However, they suffer from poor efficiency associated with the necessarily poor heat transfer between the gas and the stack. Like many thermoacoustic-Stirling engines, they also use a long resonating tube of gas to establish a resonance.
Many other oscillatory heat engines utilizing resonance exist as well, some examples of which are a cascaded thermoacoustic device, (U.S. Pat. No. 6,658,862 hereby incorporated by reference), a no-stack thermoacoustic device, (R. S. Wakeland and R. M. Keolian, “Thermoacoustics with Idealized Heat Exchangers and No Stack,” J. Acoust. Soc. Am., 111 (6), Pt. 1, 2654-2664, 2002, hereby incorporated by reference), a heat-controlled acoustic wave system of Marrison (U.S. Pat. No. 2,836,033 hereby incorporated by reference) and some very old heat engines such as singing flames, the Sondhauss tube and the Rijke tube, described by Rayleigh (J. W. S. Rayleigh, “The Theory of Sound,” Dover, N.Y., 1945, Vol. 2, pp. 224-234, hereby incorporated by reference).
The output power per unit volume (the volumetric power density) of a thermoacoustic engine may be increased by operating the engine at higher frequencies. The power output of a thermoacoustic-Stirling or standing wave engine is proportional to the engine area normal to the direction of sound, the mean pressure of the working gas, the speed of sound of the working gas, and the square of the ratio of the acoustic pressure amplitude to mean pressure of the working gas. The output power does not explicitly depend on the operating frequency of the engine. However, the overall length of the engine is proportional to the wavelength of sound, which is inversely proportional to the operating frequency, even when solids are used in place of gasses to establish resonance. Because the volume of the engine is proportional to the normal area times the engine length, it is therefore possible and advantageous to reduce the volume taken up by the engine by raising the operating frequency, which can be done with little penalty in output power. Raising the operating frequency is limited by the ability of the heat exchangers to function properly when their effective size is limited by the shorter acoustic displacements that result at higher frequencies, by the parasitic thermal conduction between hot and cold regions of the engine which will be closer together as the frequency is increased, and by the moving mass of any transducers used to exchange electrical power with the engine.
Although it has long been the practice of designers of conventional and free piston Stirling engines to use the location of the power piston for the transduction of power into or out of the heat engine, this was not originally the practice in thermoacoustics. An early attempt to shorten a thermoacoustic device was made by Hofler and Grant (L. A. Grant, “Investigation of the Physical Characteristics of a Mass Element Resonator,” M.S. Thesis, Naval Postgraduate School, Monterey, Calif., 1992, National Technical Information Service ADA251792) by substituting a resonating mass for the mass impedance presented by the typical nearly half wavelength long resonating tube. (Impedance, or more properly the specific acoustic impedance, is the complex ratio of pressure to velocity. A mass impedance has the pressure leading the velocity by 90 degrees so that the pressure is in phase with the acceleration, as would be the case for a mass.) Similarly, U.S. Pat. No. 6,314,740 and WIPO publication WO 99/20957, for the case of thermoacoustic-Stirling engines, and U.S. Pat. No. 6,578,364 (herein incorporated by reference), for the cases of both the thermoacoustic standing wave and thermoacoustic-Stirling engines, teach that when a thermoacoustic engine is used with a transducer for removing or adding power (only electrodynamic examples are shown), the length of the engine can be greatly shortened by substituting the moving mass of the transducer for the acoustic mass impedance of the working gas in the half wavelength long resonant tube. In effect, the transducer is used as the power piston of the conventional or free piston Stirling engines.
Thus, transducers that present a mass impedance may be beneficial; since the core of oscillatory heat engines often present an impedance which is primarily stiffness-like (pressure lagging velocity by nearly 90 degrees), the combination of mass-like transducers and stiffness-like engines, along with other mass and stiffness impedances as desired, may be combined to form a resonant system that result in compact useful heat engines.
The only examples of transducers that have been given in the prior art, however, that present a mass impedance which may be used for this beneficial shortening of the engine are electrodynamic, because in the prior art only electrodynamic transducers have a stroke (peak to peak displacement amplitude) sufficient to accomplish this task. Linear motion electrodynamic transducers (e.g. U.S. Pat. Nos. 4,623,808 and 5,389,844 hereby incorporated by reference) may be used for driving or generating electricity with any of the oscillatory heat engine types. This electrodynamic class of transducers, when used as alternators, for example, use various topologies to induce an electromotive force in a wire by way of a changing magnetic flux through a stationary coil, or by way of wire motion through a static magnetic field. Electrodynamic alternators, however, have the disadvantage that their moving mass tends to be large. This is not generally a problem at low operating frequencies, where it is more important that the alternator have a large stroke to roughly match the large displacement amplitude of the sound, but the large moving mass limits the use of electrodynamic transducers at high frequencies.
A number of piezoelectric generators have been proposed in the patent literature for a variety of purposes unrelated to the generation of electricity from oscillating heat engines: a vibrating reed driven by the passage of air in the nose cone of a missile (U.S. Pat. No. 4,005,319 hereby incorporated by reference), bender elements lining a automobile muffler to pull energy out of the sound of auto exhaust, (U.S. Pat. No. 4,467,236 hereby incorporated by reference), piezoelectric elements embedded in motor vehicle tires to pull energy out of the flexing of tires via slip rings (U.S. Pat. No. 4,504,761 hereby incorporated by reference), a stack of piezoelectric elements excited by the pressure pulse of an internal combustion engine (U.S. Pat. No. 4,511,818 hereby incorporated by reference), a strip of piezoelectric plastic implanted into the human body to energize implanted electronics with body movement (U.S. Pat. No. 5,431,694 hereby incorporated by reference), bender elements that are plucked by cams which move in response to ocean waves (U.S. Pat. No. 5,814,921 hereby incorporated by reference), rotary motion of an eccentric shaft that applies oscillating stresses onto piezoelectric elements in contact with the eccentric (U.S. Pat. No. 6,194,815 hereby incorporated by reference), or which are inertially stressed by the eccentric motion (U.S. Pat. No. 6,429,576 hereby incorporated by reference). Some of these applications appear to be impractical.
The use of piezoelectric alternators in place of electrodynamic alternators in a thermoacoustic application has been suggested for use with the standing wave engines (W. C. Ward et al., “Thermoacoustic engine scaling, acoustic and safety study,” Los Alamos National Laboratory unclassified report LA-12103-MS, 1991). Their alternator configuration is interesting and clever. However, like many traditional acoustic transducers, it presents to the resonator a very high impedance, and it has a very limited available stroke, factors which are associated with the high mechanical stiffness and limited mechanical strain of a raw piece of piezoelectric ceramic. The high impedance and low stroke of their transducer forces it to be placed near the pressure anti-node of the sound field where the acoustic velocity is small. Since during a cycle of sound energy oscillates between potential and kinetic forms and the transducer is incapable of holding much of the kinetic energy due to its limited stroke, the resonator needs to be about as long as a half wavelength of sound so that a large volume of fast moving gas near the velocity anti-node may accept the kinetic energy. This Los Alamos configuration of a piezoelectric alternator cannot take the space saving advantage of using the alternator as a resonating mass that may substitute for much of the gaseous mass of the acoustic resonator.
An earlier patent (U.S. Pat. No. 3,822,388 hereby incorporated by reference) showing a simple stack of piezoelectric ceramic coupled with hydraulic fluid and a column of mercury to the pressure oscillations generated by an otherwise conventional Stirling engine, also presents the engine with a transducer that is very stiff. With its high impedance and large mass of coupling fluids, it is unsuitable for use with thermoacoustic engines.
In addition to being used as prime movers, thermoacoustic and Stirling heat engines may be used in the opposite sense, accepting work in the form of the coupled pressure and volume changes of sound in order to pull heat from a thermal load at low temperature and reject heat at a higher temperature, useful for constructing refrigerators or heat pumps. Transducers used as acoustic drivers to convert electrical power into the acoustic power needed to run the refrigerators or heat pumps would potentially have the same space saving advantages as their alternator counterparts if they were made to present a mass impedance. Additionally, by combining a prime mover with a refrigerator, it is possible to make heat driven refrigerators, for example for use in remote or mobile applications where connection to the electrical power grid is not feasible or desirable. It is often necessary, however, to make electricity available for the running of fans and pumps for the distribution of the cooling effect or for auxiliary uses. It is therefore desirable to have an efficient means of generating electricity in heat driven refrigerators, chillers and heat pumps. It is also desirable to have compact transducers (alternators or drivers) that can operate at high frequency that present a mass rather than a stiffness impedance, which may be used to establish resonance with oscillatory heat engines presenting a primarily stiffness impedance. These needs are addressed by the present invention.